Plasma displays utilize emissions from regions of low pressure gas plasma to provide visible display elements. A typical display cell comprises a pair of electrodes within a sealed cell containing a noble gas. When a sufficient voltage is applied between the electrodes, the gas ionizes, forms a plasma, and emits visible and ultraviolet light. Visible emissions from the plasma can be seen directly. Ultraviolet emissions can be used to excite visible light from phosphors. An addressable array of such display cells forms a plasma display panel. Typically display cells are fabricated in an array defined by two mating sets of orthogonal electrodes deposited on two respective glass substrates. The region between the substrates is filled with a noble gas, such as neon, and sealed.
Plasma displays have found widespread applications ranging in size from small numeric indicators to large graphics dismays. Typical applications are described in H. G. Slottow, IEEE Trans. Electron Devices, Vol. ED-23, No. 7, p. 760 et seq (1976) and S. Mikoshiba, Society for Information Display, Seminar No. F-2 (1993) which are incorporated herein by reference. Plasma displays are strong contenders for future workstation displays and HDTV displays.
The commercial success of plasma displays is due to many desirable properties. For example, a plasma has very strong nonlinear current-voltage characteristic which is ideally suited for multiplexing or matrix addressing. This nonlinearity also provides internal memory and logic capabilities which can be used to reduce the number of external circuit drivers. The ultraviolet radiation from a plasma can be used to excite phosphors, thereby permitting fabrication of full color displays. Other favorable attributes of plasma displays include long lifetime (.about.10,000 hrs for dc displays and &gt;50,000 hrs for ac displays) with no catastrophic failure mechanism. They provide high resolution, good contrast ratio, a wide viewing angle (comparable to a CRT), and gray scale capability (8-bit, 256 levels). The displays are rugged, self-supporting structures which can be made in large areas (a display as large as 1.5 m diagonal with 2,048.times.2,048 pixels has been reported), and they are tolerant to harsh environment and wide temperature variations. The principal drawbacks of plasma displays are their high driver voltage (150-200 V), relatively low luminance (.about.100 cd/m.sup.2 compared to 700 cd/m.sup.2 for a CRT) and low luminous efficiency (0.2 lm/W compared to 4 lm/W for a CRT).
Plasma displays are usually classified as dc or ac. In a dc display, the electrodes are in direct contact with the plasma. The current is limited by resistance. In an ac display the electrodes are typically separated from the plasma by a dielectric, and the current is limited by capacitance.
DC displays ultimately fail because the cathode material is gradually sputtered or eroded away under the bombardment of positively charged energetic ions from the plasma. Erosion or sputtering of these cathode materials limits the typical lifetime of a dc plasma display at .about.10,000 hours. The sputtering also leads to the deposit of cathode material on the inner surface of the enclosing glass envelope, reducing the transmission of light.
Addition of small amounts of mercury reduces the sputtering problem but does not solve it. Although the addition of mercury in the gas reduces the effect of sputtering by several orders of magnitude, mercury particles tend to condense at the coldest spot. As a result, active regions where sputtering is severe have less mercury. Mercury is also chemically reactive with metals such as Ba and Ag which are used as electrode or electric lead materials. In addition, the strong visible emission from mercury degrades the color purity.
AC displays using conventional materials are subject to problems of contamination. In a typical ac plasma display the conductive electrode is covered by a dielectric layer which is, in turn, overcoated with MgO. The MgO overcoating has a high secondary electron emission coefficient which reduces the breakdown voltage for the gas. In addition, MgO is resistant to sputtering and thus gives the device a very long lifetime. The problem is that MgO is susceptible to contamination in the manufacturing process. Once contaminated, it is virtually impossible to clean.
The high operating voltage (150-200 V) in conventional plasma displays is disadvantageous. The use of relatively high operating voltages and associated problems in dielectric breakdown make it necessary to use tall dielectric barrier ribs between the cathode and the anode. Since much of the energy loss in the plasma displays is due to the collision of the plasma with the barrier ribs, high aspect ratio display cells with large surface to volume ratios are not desirable. In addition, higher pixel-density displays with smaller cell sizes are difficult to obtain if the barrier rib is to stay tall.
If the operating voltage can be lowered, the height of the rib can be reduced and hence smaller cell sizes can be implemented. Shorter ribs would increase the solid angle subtended by the front transparent electrode and reduce the number of photons absorbed by the barrier rib. Thus for a given input power, more photons would exit the display.
Accordingly, there is a need to develop new electrode materials for both dc and ac plasma displays which will provide low operating voltage, mechanical robustness, chemical stability and tolerance to harsh environment.